In an ink-jet printing method, droplets of an ink are discharged directly onto a recording medium from very fine nozzles and allowed to adhere to the recording medium to form characters and images. The above method is widely used in recent years because of not only the advantage that an apparatus used generates low noise and has a good operating property but also the advantages that colorization is easy and that plain paper can be used as a recording medium.
In recent years, inks (hereinafter referred as “pigment base inks”) using a pigment or a hydrophobic dye as a colorant are mainly used for inks used for ink-jet printers from the viewpoint of a water resistance and a weatherability.
When a pigment base ink using a polymer is used for ink-jet printing of a thermal system, discharge stability, storage stability and heat stability of higher levels are required as compared with the case of a piezo system.
It is considered to be effective as means for elevating a storage stability of pigment base inks to raise a dispersibility of a pigment by a polymer. Thus, macromers and graft polymers having a long chain alkyl group are used as a structural component for a polymer.
Known as the macromers are styrene-homopolymerized macromers having a polymerizable functional group at one end or copolymerized macromers of styrene with other monomers, silicon macromers and alkyl (meth)acrylate macromers (refer to, for example, Japanese Patent Application Laid-Open No. 100810/1994, Japanese Patent Application Laid-Open No. 247796/2001, Japanese Patent Application Laid-Open No. 338783/2002, Japanese Patent Application Laid-Open No. 255870/2004, International Publication No. 00/39226, Japanese Patent Application Laid-Open (through PCT) No. 526259/2002 and Japanese Patent Application Laid-Open (through PCT) No. 517063/2003).
However, when conventional water-based inks are used for ink-jet printing of a thermal system, the heat stability thereof have not sufficiently been satisfactory. A reduction in a heat stability of the inks causes clogging in the nozzles of a printer and causes as well a reduction in a discharge stability thereof.
Methods in which a molecular weight of graft polymers is raised and in which a copolymerization amount of macromers in the above polymers is increased are considered as means for elevating a heat stability of a pigment base ink. However, this causes an increase in a viscosity of the water dispersion, which leads in turn to an increase in a viscosity of the water-based ink, and the water-based ink is reduced in a discharge stability to bring about problems such as starving of the printed letters.